When I try painting a texture with the fill brush i get a weird result
Umm not sure if this is a bug of blender. Because I had the same issue too,  so a workaround is using the uv image editor and painting your texture on the image editor directly, avoid painting throught the 3d viewport blender doesnt like it, dont know why. :)

To enable painting on the image editor only change the tab that says view to paint. 

Goodluck